Is there a way in Ubuntu 18.04.3 LTS that once a tab of "Terminal" window is detached from that window, to re-attach that tab to the another already opened Terminal window (with other tabs) again?
No, unfortunately it is one way only. You can detach a tab into a new window, but you cannot attach a terminal window as a tab to another terminal window.
The trick described by this answer doesn't work in Gnome Terminal 3.28.2 in Ubuntu 18.
However, I just noticed that the same trick does work in MATE terminal 1.24.1 on Debian 11. Detaching a terminal is performed by drag and drop, and if we create a dummy tab in the detached terminal, the original becomes a tab that we can drag and drop into the original window to reattach. Note: this operation seems to need the actual terminal payload window as a target, not the tab area above it. Dropping into the tab area will detach it, same as dropping into the desktop.
I originally installed MATE on Debian because of a copy and paste problem: text copied and pasted from a Gnome terminal running in a guest VirtualBox turned double spaced (extra newlines) when pasted into anything in the host.
Looks like MATE provides a workaround for this reattach problem as well.
MATE is a fork of Gnome Terminal from the MATE project, which is a continuation of Gnome 2.
You have your original terminal (from where your TAB was detached) - let's call it T1.
You have the detach TAB (which is actually a new terminal) - let's call it T2.
In T2 open a new TAB - now you have two TABs in T2.
Now you can detach the TAB you need by Drag & Drop it back to T1.
No, unfortunately it is one way only. You can detach a tab into a new window, but you cannot attach a terminal window as a tab to another terminal window.
The trick described by this answer doesn't work in Gnome Terminal 3.28.2 in Ubuntu 18.
However, I just noticed that the same trick does work in MATE terminal 1.24.1 on Debian 11. Detaching a terminal is performed by drag and drop, and if we create a dummy tab in the detached terminal, the original becomes a tab that we can drag and drop into the original window to reattach. Note: this operation seems to need the actual terminal payload window as a target, not the tab area above it. Dropping into the tab area will detach it, same as dropping into the desktop.
I originally installed MATE on Debian because of a copy and paste problem: text copied and pasted from a Gnome terminal running in a guest VirtualBox turned double spaced (extra newlines) when pasted into anything in the host.
Looks like MATE provides a workaround for this reattach problem as well.
MATE is a fork of Gnome Terminal from the MATE project, which is a continuation of Gnome 2.
You have your original terminal (from where your TAB was detached) - let's call it T1. You have the detach TAB (which is actually a new terminal) - let's call it T2.
In T2 open a new TAB - now you have two TABs in T2.
Now you can detach the TAB you need by Drag & Drop it back to T1.
Then you can kill T2. :)